In the past, a technique of detecting spatial information by use of an intensity-modulated light has been utilized. That is, the intensity-modulated light is irradiated from a light source into a space, and a light reflected from an object in the space is received by a photoelectric converter. The spatial information can be determined from a relationship between the intensity-modulated light and the received light. In the present description, the spatial information includes a distance from the object, a change in light receiving amount caused by reflections on the object, and so on. For example, the distance information can be determined from a phase difference between the intensity-modulated light and the received light. In general, this technique is called as “Time-of-Flight (TOF)” method.
For example, U.S. Pat. No. 5,856,667 discloses a distance measuring apparatus using the TOF method. In this apparatus, a light emitted from a light source is intensity-modulated by a sine wave having a required modulation frequency (i.e., emission frequency), and a light sensitive part detects an intensity of received light plural times within a time period shorter than a modulation period that is a reciprocal of the modulation frequency. When detecting the intensity of received light is repeated 4 times within one modulation period, the phase difference is determined from the detected four intensities of received light. For example, when intensity-modulating the light irradiated from the light source to an object by a radio frequency wave of 20 MHz, a wavelength of the intensity-modulated light is 15 m. Therefore, when the intensity-modulated light goes to the object spaced from the apparatus by a distance of 7.5 m and back, a phase delay corresponding to one modulation period occurs.
When the light emitted from the light source is intensity-modulated, as shown by the curve “W” of FIG. 30, and the modulated light reflected from an object is received by the light sensitive part, the intensity of received light changes, for example, as shown by the curve “R” of FIG. 30. In this case, four intensities (A0, A1, A2, A3) of received light can be detected at 4 different phases (0°, 90°, 180°, 270°). However, in the present circumstances, it is impossible to detect the intensity of light received at just the moment of each of the phases (0°, 90°, 180°, 270°). In the fact, each of the intensities of received light corresponds to the intensity of light received within a time width (Tw), as shown in FIG. 30.
On the assumption that the phase difference “ψ” does not change within the time period of sampling the intensity (A0, A1, A2, A3) of received light, and there is no change in light extinction ratio between the emitted light and the received light, a relationship between the intensity (A0, A1, A2, A3) of received light and the phase difference “ψ” can be represented by the following equation:ψ=tan−1{(A3−A1)/(A0−A2)}.
By use of the thus obtained phase difference “ψ” [rad], the modulation period “T” [s], and the speed of light “c” [m/s], a distance “L” [m] between the object and the apparatus can be calculated by the following equation:L≈cT(ψ/4π).
To achieve the technical concept described above, this US patent proposes to use an image sensor shown in FIG. 31, which comprises four memory cells (M0, M1, M2, M3) provided every light sensitive part (PD), and an electrical switch (S0, S1, S2, S3) disposed between each of the memory cells and the light sensitive part. The electrical switches (S0, S1, S2, S3) are respectively turned on within the short time width (Tw), as shown in FIG. 30, to store the intensities (A0, A1, A2, A3) of received light in the memory cells (M0, M1, M2, M3). By repeating this procedure with respect to a plurality of periods, it is possible to reduce influences of dark current noises, shot noises (i.e., noises caused by variations in the occurrence of electron-hole pairs), static noises of an amplifier circuit, and so on, and improve S/N ratio. In the present specification, the operation described above is called as “synchronized integration”.
However, when electric charges are generated by the light sensitive part, parts of them may remain in the light sensitive part for a moment without being transferred to the memory cell through the electric switch. Such residual electric charges disappear by recombinations in the light sensitive part. Alternatively, when another electrical switch is turned on within the time width (Tw), the residual electric charges may be accidentally transferred to another memory cell through to the electric switch.
For example, when the modulation frequency is 20 MHz, it is needed that the time width (Tw) is shorter than the modulation period of 50 ns. On the other hand, time required for allowing the residual electric charges to disappear by the recombinations is usually longer than 100 μs. Therefore there is a possibility that the residual electric charges are accidentally transferred to another memory cell. This means that the residual electric charges are mixed as noise components in signal charges to be stored in the memory cell. As a result, when determining the phase difference “ψ” according to the above-described operation, there is a problem of lowering the accuracy of detecting the distance information. In addition, when using the image sensor with a large number of electrical switches, as shown in FIG. 31, there is another problem deteriorating cost/performance of the distance measuring apparatus.